[Feature Request] Consider supporting singleton pattern in recursive instantiation

The following config can in theory be used to instantiate one object
```yaml

container:
  _target_: ...
  object1:
    _target_: ...
    _singleton_: true

  object2: ${.object1}
```

```python
c = instantiate(cfg.container)
assert c.object1 is c.object2
```

This will will probably require changes in OmegaConf.